Railway hazard detection systems have been employed for many years, to detect track discontinuities, such as breaks or severe misalignments. Prior systems commonly loop an electrical circuit through a portion of track and detect a discontinuity in the track by a corresponding break in the circuit. However, these detection systems are usually inadequate to detect certain hazards occurring on a railway, such as landslides and falling boulders that impinge on but do not break or misalign the tracks. Accordingly, additional systems are usually employed to detect these hazards at locations at which they are likely to occur.
One manner of landslide detection involves stringing a number of current-carrying wires up-hill from the railway. When a falling rock, or similar object, breaks a wire, the flow of current through the wire stops and an alarm circuit is tripped. This type of detection system has several drawbacks. First, it may be expensive and time consuming to install the various wires, especially in locations with rugged terrain. Second, depending upon the relative spacing of the wires, some objects may be able to pass through, over, or under the wires and thereby evade detection. Third, after a break occurs, extensive repairs may be necessary to restore the integrity of the wire network.
Another prior landslide detection system relies on changes to an electromagnetic field established between two graded, "leaky" cables to detect an intrusion. As shown in FIG. 1, two coaxial cables, namely, a transmit cable 2 and a receive cable 4, are deployed at a constant distance from each other along an area of interest. The cables are specially manufactured so that their shields have a plurality of slots 8 along their lengths, to allow a small amount of signal energy to couple from the transmit cable 2 to the receive cable 4. To compensate for signals loses along the cables, the separations between the slots are decreased and sizes of the slot openings are increased with distance from the transmitter and receiver. For intrusion detection, the electromagnetic field associated with the cables must be essentially uniform along the cable length. Accordingly, the relative sizes and positions of the slots are critical. Further, terminators 6 having an impedance that is matched to the characteristic impedance of the cables are attached to the far end of each cable, to minimize signal reflections and standing waves that may produce false readings.
Under steady state, i.e., non-intrusive, conditions, the electromagnetic field 7 is static and the received signal has a constant magnitude and phase. When an object comes near either cable, or between the cables, the electromagnetic field is perturbed, causing a change in the magnitude and/or phase of the received signal.
Unfortunately, the use of graded, leaky coaxial cable in such systems has some significant drawbacks. First, because the size and spacing of the slots can only be adjusted by a finite amount before affecting the characteristic impedance of the cables, there is a fundamental limitation on run length. Second, the cables must be special-ordered and are expensive to manufacture. Third, great care must be taken during installation to properly orient the cables, so that an essentially constant field may be achieved. Lastly, repairs to damaged cable present special problems. While short sections of graded cable may be repaired using a standard patch kit, the replacement of a longer section is difficult because the new cable section must have slots that are properly spaced and sized, to avoid a significant impact on performance. Accordingly, a relatively large inventory of graded cable sections must be maintained.